Effect of the Surface Electric Potential on Sensitized Photolysis of Lead Azide

Yu. V. Gavrishchenko, G. G. Savel’ev, Yu. A. Zakharov

The existence of a correlation between photolysis and the photoconductivity of inorganic compounds is often used to clarify the mechanism of photolysis. However, in the study of sensitized samples, as well as of photographic emulsions, such a correlation is not always observed. The causes of this phenomenon have not yet been studied, since the presence of gelatin makes it difficult to interpret the results of studies of photographic emulsions; and in studies of sensitized silver bromide samples, the bromine released as a result of photolysis destroys the adsorbed dye.

We have chosen the system heavy-metal azides—organic dyes. Such a system is more convenient for investigation than the classical system silver bromide—organic dye, since the final products of azide photolysis, metal and nitrogen, are only slightly active and do not destroy organic dyes.

Photolysis of heavy-metal azides has the following general scheme:

\[ \mathrm{N}_3^- \xrightarrow{h\nu} \mathrm{N}_3^0 + e \]

\[ \mathrm{Me}^+ + e \to \mathrm{Me} \]

\[ 2\mathrm{N}_3^0 \to 3\mathrm{N}_2 \]

That is, the metal ion \(\mathrm{Me}^+\) liberated as a result of ionization attaches an electron and forms a metal atom. For nitrogen formation, the radical \(\mathrm{N}_3^0\) (a hole) must also emerge at the surface of the crystal, where it interacts with another radical.

We have previously shown that lead azide has hole-type conductivity [1]. For this reason, an antibarrier surface potential should promote the motion of holes toward the surface and increase the probability of the bimolecular reaction that is the final stage of photochemical decomposition.

To elucidate the mechanism of sensitized photolysis, we carried out studies of photochemical decomposition, photoconductivity—

conductivity and photopotential of pure lead azide and of lead azide sensitized with organic dyes.

The rate of photochemical decomposition was determined by a vacuum technique using the compensation method of pressure measurement [2]. The methods for measuring photoconductivity and photopotential—

Fig. 1. Photoinduced potential of pure lead azide (1) and of lead azide colored with erythrosine (2), sodium eosin (3), cryptocyanine (4), and trypaflavine (5)

Fig. 2. Photoinduced potential of lead azide colored with indocarbocyanine (1) and thiocarbocyanine (2)

Fig. 3. Spectral distribution of the photolysis rate of pure (1) and lead azide dyed with erythrosin (2), sodium eosin (3), and cryptocyanine (4).

were similar to those described in [3]. From measurements of the spectral distribution of the photopotential, the sign of the surface potential was determined [4].

The investigations showed that erythrosin, sodium eosin, cryptocyanine, thiocarbocyanine, and indocarbocyanine sensitize the photoconductivity of lead azide. When lead azide was dyed with trypaflavine, no sensitization was detected.

Figures 1 and 2 present the results of measuring the spectral distribution of the photopotential, which indicate that erythrosin, sodium eosin, cryptocyanine, and trypaflavine create an anti-blocking surface potential, whereas thiocarbocyanine and indocarbocyanine create a blocking surface potential in lead azide.

The spectral distribution of the photolysis rate is shown in Fig. 3. Sensitization of photolysis was observed only when the samples were dyed with erythrosin, sodium eosin, and cryptocyanine.

The measurement results show that sensitization of photoconductivity is a necessary but not sufficient condition for sensitization of photolysis, since sensitization of photolysis also requires the presence of an anti-blocking surface potential.

The study of the spectral distribution of the photolysis rate showed that, as the wavelength of the incident light is decreased in the intrinsic absorption region, the photolysis rate decreases despite the increase in the energy of the light quanta. These results agree with measurements of the spectral distribution of photoconductivity, where photoconductivity decreases because of an increase in the rate of surface recombination as the absorption coefficient increases. It is possible that the decrease in the photolysis rate is also determined by a decrease in the concentration of \(N_3^0\) radicals owing to an increase in the rate of their surface recombination with electrons.

The investigation carried out demonstrates the important role of the electrical state of the surface in the photolysis of heavy-metal azides.

References

1. G. G. Savel’ev, Yu. V. Gavrishchenko, Yu. A. Zakharov, Izv. VUZov, “Fizika,” No. 7 (1968).
2. Yu. V. Gavrishchenko, G. G. Savel’ev, Izmer. tekhn. (in press).
3. A. M. Meshkov, I. A. Akimov, PiTE, No. 3, 5 (1964).
4. I. A. Akimov, Optiko-mekh. prom., No. 5, 10 (1966).